THE EARTH. Si 



at some vast distance under the earth, large quantities of 

 inflammable matter, pyrites, bitumens, and' marcasites, dispos- 

 ed, and only waiting for the aspersion of water, or the hu- 

 midity of the air, to put their fires in motion : at last, this 

 dreadful mixture arrives -, waters find their way into those 

 depths through the perpendicular fissures ; or air insiimates it- 

 self through the same minute apertures : instantly new appear- 

 ances ensue ; those substances, which for ages before lay dor- 

 mant, now conceive new apparent qualities ; they grow hot, 

 produce new air, and only want room for expansion. However, 

 the narrow apertures by which the air or water bad at first ad- 

 mission are now closed up ; yet as new air is continually gener- 

 ated, and as the heat every moment gives this air new elasticity, 

 it at length bursts, and dilates all round ; and, in its struggles 

 to get free, throws all above it into similar convulsions. Thus 

 an earthquake is produced more or less extensive, according to 

 the depth or the greatness of the cause. * 



• The existence of volcanoes and hot springs led philosophers long ago to 

 suspect that there was au inteuse heat ia the interior of the earth, 'i'he 

 opinion of Werner, that the former arose from the combustion of masses of 

 coal .It moderate depths, was set aside by the discovery that the seat of the 

 volcanic agents was under the primitive rocks, of course far below the coal, 

 formation, and that the composition of lavas was the same in all parts of the 

 world. The notion advanced by others, that hot springs might owe their 

 origin to the accidental mixture of substances producing chemical action in 

 the bowels of the earth, was equally inadequate to account for the per- 

 manency of these springs — their existence without any known change for 

 ages. At length a tliird species of evidence presented itself in tiie tempera- 

 ture of deep mines, which it was observed was generally higher than the 

 mean temperature of the year in the district. It was objected that the heat 

 might arise from the breaths of the workmen, and the lights used by them. 

 This explanation to be sure did not account tor the difference of temperature 

 Enid to be observed between shallow and deep mines ; but the existence of 

 the difierence alluded to was doubted j and to this as the point upon which 

 the controversy hinged, several philosophers, especially M. Cordier, a pro. 

 fe&sor of geology in Paris, directed their attention. The result is thus an 

 nouuced by the Parisian professor : — " I. Our experiments fully confirm the 

 existence of a subterranean heat, wiiich is peculiai' to the terrestrial globe, — 

 does not depend on the solar rays,— and increases rapidly with the depth. 



2. The increase of the subterranean heat does not follow the same law over 

 the whole earth ; it may be twice or thrice as great in one country as another. 



3. These differences do not bear any constant proportion either to the lati- 

 tude or longitude. 4. The increase is more rapid than has been supposed ; it 

 may go as high as one degree of Fahrenheit for 2i feet, but the mean, so far 

 ia the present observations have yet extended, cannot be fi.\ed at less than 



